Paint compositions especially adapted for the coating of glass require a number of specific properties for commercial acceptance. The paint compositions must have the required color and opacity, and the coatings made from such compositions must have suitable resistance to the environment. These include scratch resistance, and resistance to acids and alkalis. The paint compositions should also be readily usable in a variety of printing mediums including UV-curable, water-miscible, thermoplastic, and conventional screen printing mediums.
Traditional glass coatings utilize lead oxide as a major component of the flux. From an engineering standpoint, the presence of lead oxide is advantageous. It provides for a smooth surface, excellent gloss, and low melting temperatures. Since lead oxide is a very powerful flux component, it minimizes the need for auxiliary components such as alkali oxides. Generally, alkali oxides should be minimized because they tend to raise the coefficient of thermal expansion (CTE) of the glass. If the CTE of the coating surpasses that of the substrate (e.g. soda lime silicate glass), tensile stress of the coating is increased during the cooling stage of the firing process. Since glass is very weak in tension, elevated tensile stress leads to fracture or "crazing" of the glass.
Notwithstanding the advantages of lead oxide, the health and environmental hazards of lead oxide-containing materials favor their elimination and the development of suitable flux and paint compositions that are free of such materials. There is therefore a significant need in the art of coating glass substrates for lead oxide free coating compositions which are highly resistant to scratching, and corrosive materials such as acids and alkalis.